As standard, a Bentech frame is built to take a 26 inch rear wheel.
It's hard to get a 26 inch wheel with a narrow rim (at least in Europe)-
the narrowest I've ever found is 1.5 inch. If you really want to go
fast, it might be worth asking Dom to build it to take a 700C rear
wheel. Then you can fit really light, narrow, fast racing tyres.

Then again, there's a school of thought that says a fat tyre on a
smaller wheel, pumped up hard, has a smaller footprint than a narrow
tyre on a larger wheel, and so goes faster...

I think it's fair to say too that Bentechs aren't the lightest or the most aero
'bents around - if it's all out speed you're after, something from Challenge or
Toxy might be nearer the mark... but more money of course.